Differences in the electrical charge across the membrane of neuronal cells provide one of the bases for communication between individual neurons in a network. In a normal biological system, these differences are achieved by passive and dynamic process involving, among others, ion channels and pumps that ensure an uneven distribution of negatively and positively charged ions (mainly sodium (Na+), potassium (K+), chloride (Cl−), and calcium (Ca2+)) on each side of the membrane, such that the cell is said to be polarized. The inside of the cell under resting conditions is more negative than the outside and changes in the movement of ions across the membrane render the cell even more negative (hyperpolarized) or less negative (depolarized). Changes in the movement of ions thus result in a change in the membrane potential of the neuronal cell and can be triggered, directly or indirectly, by e.g., the binding of ligands to specific membrane receptors, mechanical forces, temperature, or light.
A change of the membrane potential can also be achieved by a direct electrical stimulation of neurons, a technique which is widely used in neuroscience. Applications range from basic studies of biological neural networks to medical applications, such as deep brain stimulation or retinal implants.
The loss of specific neuronal cells in the brain leads not only to a loss of function but also to an imbalance in excitatory and inhibitory signals (which can involve several neuronal circuits) and are the cause of or contribute to the symptoms of several disabling neurological (e.g., Parkinson's Disease) and psychiatric disorders. Deep brain stimulation (DBS), also referred to as focal brain stimulation (FBS), is a form of electrotherapy used clinically to treat many of the symptoms observed in these diseases. It uses surgically implantable electrodes to stimulate a neuron or neural network in the brain through direct or indirect excitation of the cell membrane with an electric current or electric potential. The generated electrical impulses modulate neuronal activity, reducing some of the symptoms.
Visible electromagnetic radiation induces an activation of photosensitive proteins in specialized cells in the retina (rod and cone photoreceptors). The activation of these proteins leads to a change in the flow of ions across the photoreceptor cell membrane, which in turn determines the amount of neurotransmitter released by these cells. Genetic and acquired diseases, as well as trauma, can cause the death and loss of retinal photoreceptors leading to visual impairment and eventually complete blindness. Synthetic photosensors may be used to replace the function of defective biological rods and cones, providing light-induced electrical stimulation to the visual nervous system.
In a normal retina, the signals generated by the photoreceptors are passed on to bipolar cells and subsequently to retinal ganglion cells, which ultimately convey the visual information through the optic nerve to higher visual centers. Degenerative diseases and trauma can lead to a loss of axons in the optic nerve and loss of the ganglion cells, leading to severe visual impairment.
Neuroprotection refers to any strategy used to delay or prevent neuronal cell death. Depolarization of neuronal cells by e.g., increased extracellular K+ or direct electrical stimulation have been shown to increase the survival of several neuronal cell types.
Conventional electrical stimulation devices use an external electric power supply to power the device. Even conventional artificial photosensitive cells, e.g., conventional retinal implants, use an external power supply to amplify the signal and/or make the device functional.